The present invention relates generally to a superconducting magnet and more particularly to a superconducting magnet whose superconductive coil is generally surrounded by a thermal shield.
Superconducting magnets are used, or are planned to be used, in various apparatus such as, but not limited to, magnetic resonance imaging (MRI) systems for medical diagnosis, superconductive rotors for electric generators and motors, and magnetic levitation devices for train transportation. Conventional superconductive magnets include at least one superconductive coil surrounded by a 60-250 thousandths-inch-thick rigid thermal shield surrounded by a sturdy vacuum enclosure. The rigid thermal shield, which usually is made of aluminum or copper, reduces heat transfer to help maintain a low cryogenic temperature in the superconductive coil and requires a sturdy support so as to keep the thermal shield spaced-apart from the superconductive coil and the vacuum enclosure to reduce heat transfer. Certain vacuum enclosures for MRI systems have a generally toroidal shape with an open bore and are generally coaxially aligned with the superconductive coil, as are known to those smiled in the art.
It is known to use thermally-insulative rigid spacers in superconductive magnets between the thermal shield and the vacuum enclosure, and between the superconductive coil and the thermal shield, to securely position such magnet components. It is also known to use support members, in place of spacers to attach such magnet components together. A typical vacuum enclosure for a superconductive whole-body MRI magnet may weigh 3,000 pounds and have a diameter of 82 inches, and an associated typical thermal shield may weigh 400 pounds.
Some superconductive magnets are conductively cooled by a cryocooler coldhead (such as that of a conventional Gifford-McMahon cryocooler) whose housing is hermetically connected to the vacuum enclosure, whose first stage extends from the housing into the vacuum enclosure to be in thermal contact with the thermal shield, and whose second stage extends from the first stage to be in thermal contact with the superconductive coil.
Other superconductive magnets are cooled by a dewar containing a liquid cryogen (such as liquid helium) in which is placed the superconductive coil. Two spaced-apart thermal shields surround the dewar, and a vacuum enclosure surrounds the thermal shields. To reduce liquid helium boil-off, it is known to add a cryocooler coldhead whose housing is hermetically connected to the vacuum enclosure, whose first stage is in thermal contact with the outer thermal shield, and whose second stage is in thermal contact with the inner thermal shield.
It is noted that the outer thermal shield is cooled by the first stage of the cryocooler coldhead to reduce heat transfer across the thermal shield, as is well within the understanding of the artisan. It is known to use a blanket of multi-layer insulation, such as crinkled layers of aluminized mylar, between the thermal shield and the vacuum enclosure to further reduce heat transfer.
What is needed is a better-insulated superconductive magnet which is smaller in size, lighter in weight, and lower in cost than conventional magnets.